1. Field of the Invention
The present invention relates to feeding systems and in particular to a drive system for a volumetric feeder.
2. The Related Art
Automated feeding systems for bulk solids are used for process control in a variety of industrial applications. Automated feed systems generally include a feed hopper for containing the material and a feeder for metering out a desired amount of the material per unit of time. Feeder systems generally are divided into two types: volumetric and loss-in-weight feeders.
In volumetric feeders, the volumetric flow rate is controlled. These feeders are advantageous because of their simple construction. An example is the volumetric screw feeder, in which the flow rate is varied as a function of the speed of at least one motorized horizontal screw. Under its own weight, material in the feed hopper is forced into the threads of the feed screw. As the screw turns, it pushes the material through a restrictive tube, to the downstream equipment. Many screw feeders have more than one feed screw to handle sticky materials.
Loss-in-weight (or gravimetric) feeders may be more appropriate where material density is not uniform, the consistency varies, or great accuracy is required. A loss-in-weight feeder may be formed by adding a scale and an appropriate process controller to a volumetric feeder. The control equipment compares the actual amount of material discharged with a preselected setpoint and adjusts the volumetric flow as appropriate. The control equipment typically makes loss-in-weight feeders more costly than volumetric feeders.
In order to most accurately control the material flow rate in a volumetric screw feeder, it is desirable to closely control the speed of the feed screw. One method of controlling the speed of the screw is to vary the motor speed. Relatively inexpensive motor controllers can accurately reduce the motor speed from its maximum value by a factor of 20:1 (referred to as the turndown ratio). While more accurate controllers can accurately reduce the motor speed to 1/100 of its maximum value, these controllers are more expensive. Furthermore, it may be desirable to accurately reduce the screw speed to less than 1/100 of the maximum motor speed.
More accurate speed control has typically been accomplished by the use of a gear box to vary the number of screw rotations per rotation of the motor shaft. Although the gears are accurate and reliable, they also have disadvantages. To prevent the gears from wearing when operated at high speeds, the gear box is sealed and filled with oil. In order to change the gear ratios, the gear box is mechanically removed and a different gear box is substituted. This gear-changing operation is labor intensive and makes the system more vulnerable to human error.